March 28, 18 78 J 



NA TURE 



427 



perhaps but a semitone's alteration of pitch, is often most 

 surprising. It was these sudden kaleidoscopic bounds 

 from one form to another which suggested the name given 

 to the observing instrument. In general the complexity 

 of the figure increases with the acuteness of the exciting 

 sound. With low notes a comparatively simple arrange- 

 ment of a few rings and pairs of vortices occupies the film. 

 As the pitch rises, the separate parts of the figure diminish 

 in size and increase in number, so that the whole field is 

 covered with a regular pattern which is constantly grow- 

 ing more and more minute. With very shrill sounds the 

 pattern can only be made out by using a magnifying- 

 glass. 



Effects of Quality.— These are easily observed by em- 

 ploying unison organ-pipes of different timbres, e.g., 

 treble C's belonging to stopped and open diapasons, 

 claribella, and hautbois, respectively. By sounding them 

 consecutively in the above order, figures rapidly increas- 

 ing in complexity are obtained. 



Prominent among differences of quality are those which 

 distinguish vowel- sounds of the human voice sung suc- 

 cessively on one and the same note. Marked corre- 

 sponding differences of colour-figure are recognisable 

 in many instances, but I have not at present succeeded 

 in extending the observation to all the European vowel- 

 sounds. 



Effects due to Direction of Vibration. — The best mode 

 of observing these is to strike a tuning-fork, and hold it 

 with one of its prongs close to the surface of the film. 



By moving the fork it is easy to show that both the axis 

 of symmetry, and to some extent also the form, of the 

 colour-figure thus produced, are dependent on the position 

 of the fork with respect to the film, and therefore on the 

 direction in which the exciting vibrations impinge upon 

 it. The steady bands of a figure obtained by this method 

 shift to and fro upon the film in obedience to the fork's 

 movements, almost as though under a magnetic influence 

 resident in its prongs. 



Resultant Figures due to Combined Sounds. — If the 

 sounds of two tuning-forks are separated by a considerable 

 interval of pitch, say an octave, they will generate, when 

 alternately applied to the same film, very different figures. 

 When both are applied together there results a figure dif- 

 ferent from either of those due to each fork by itself. It 

 is in fact a compromise between the two. In order to con- 

 vince himself of this the experimentor should first get the 

 forms of the component figures well into his memory by 

 repeatedly producing them, and then watch the effect, 071 

 some »ne band in either figure, of mixing the two sounds 

 in various degrees of relative intensity. Let us suppose 

 that fork i produces figure i, and fork 2 figure 2, respec- 

 tively, and that a band in figure i is selected for observa- 

 tion. Then if fork i be struck sharply, and fork 2 

 weakly, the band will alter its form so as to exhibit a 

 slight approach to the arrangement in the corresponding 

 part of figure 2. As the note of fork 2 is more loudly sounded 

 this approach will be more decided. If fork 2 is made 

 preponderant the result will be the arrangement of Fig. 2 

 with some modification towards that of figure i . The same 

 thing holds good for the rotating portions of the figures. 

 Complex colour-flows are seen to result from a com- 

 promise between simpler component vortices. 



Effect of Beats. — When two sounds of very nearly the 

 same pitch coexist, slow fluctuations of intensity called 

 " beats " are known to be produced. If a film is exposed 

 to the simultaneous action of two sounds so related, the 

 fixed parts of the resulting figure take up a swaying 

 motion about their mean position, each complete oscilla- 

 tion synchronising exactly with one entire beat. The 

 vortices show, in general, an increased speed of rotation 

 during one half of each beat, and a diminished speed 

 during the other half. But in particular cases a bolt 

 forward every alternate half-beat seems to be followed by 

 intermediate quiescence, or the direction of motion may 



be actually reversed, so that a vortex rotates positively 

 during one half-beat and negatively during the next. 



Representation of Dissonance. — When the beats become 

 too rapid for separate recognition, and coalesce into the 

 effect which we call discord, the colour-figure presents a 

 tremulous appearance, like that shown by the tip of a 

 singing gas flame. Prof. Helmholtz has remarked how 

 unpleasant is the impression which a flickering light 

 makes upon the eye, and pointed out its analogy to the 

 effect of rapidly intermittent sounds on the ear. In the 

 present experiment, acoustical and optical dissonance are 

 exhibited in a direct and interesting connection. 



As the phenomena described in the above article admit 

 of such facile reproduction in all their beauty of form and 

 splendour of hue, I have thought it needless to attempt 

 illustration by diagrams, which could convey but an 

 inadequate notion of the former, and none at all of the 

 latter. Sedley Taylor 



Trinity College, Cambridge, March 6 



REFLECTION OF LIGHT ^ 



■pLACE the heliostat in position, and bring a slender 

 ^ beam of light into the darkened room. Then get a 

 small looking-glass, or hand-mirror, and a carpenter's 

 steel square, or a sheet of stiff paper, having perfectly 

 square corners. Hold the mirror in the beam of light. 

 At once you see there are two beams of sunlight, one from 

 the heliostat and another from the mirror. Hold the 

 glass toward the heliostat, and you will see this second 

 beam going back toward the window. 



This is certainly a curious matter. Our beam of light 

 enters the room, strikes the mirror, and then we appear to 

 have another. It is the same beam, thrown back from 



■ •, X 



Fig. X 



the glass. This turning back of a beam of light we call 

 the reflection of light. 



Place a table opposite the heliostat, and place the 

 mirror upon it, against some books. Turn the mirror to 

 the right, and the second or reflected beam of light moves 

 round to the right. Turn the glass still more, and the 

 beam of light will turn off at a right angle, and there will 

 be a spot of light on the wall at that side of the room. 

 Now bring the carpenter's square or the piece of square 

 paper close to the mirror, so that the point or corner will 

 touch the glass just where the sunlight falls upon it. Now 



' From a forthcomiDg volume of the " Nature Series " — " Light : a Series 

 of Simple, Entertaining, and Inexpensive Experiments in the Phenomena 

 of Lisht, for the Use of Students of Every Age," by Alfred M. Mayer and 

 Charles Barnard. 



